There have been conventional methods for estimating a road surface condition by detecting tire vibration during vehicular travel. In such a method, a detected time-series waveform of tire vibration is divided into a plurality of regions, such as “pre-leading-end region-contact area region-post-trailing-end region” or “pre-leading-end region-leading-end region-contact area region-trailing-end region-post-trailing-end region”. And from these regions, vibration levels in certain frequency ranges where the vibration level changes markedly for different road surface conditions, such as the vibration component in a low-frequency range and the vibration component in a high-frequency range in the pre-leading-end region and the contact area region, for instance, and vibration levels in frequency ranges where the vibration level does not change with different road surface conditions are extracted. Then the condition of the road surface on which the vehicle is running is estimated from the ratio between these vibration levels (see Patent Document 1, for instance).